Prevent triggering pipelines when target branch is updated

Currently, pipelines for merge requests are triggered when source or
target branch is updated. However, we should create only when source
branch is updated, because it runs unexpected pipelines.

app/services/merge_requests/refresh_service.rb

@@ -21,6 +21,7 @@ module MergeRequests
       post_merge_manually_merged
       reload_merge_requests
       outdate_suggestions
+      refresh_pipelines_on_merge_requests
       reset_merge_when_pipeline_succeeds
       mark_pending_todos_done
       cache_merge_requests_closing_issues
@@ -107,8 +108,6 @@ module MergeRequests
         end
 
         merge_request.mark_as_unchecked
-        create_pipeline_for(merge_request, current_user)
-        UpdateHeadPipelineForMergeRequestWorker.perform_async(merge_request.id)
       end
 
       # Upcoming method calls need the refreshed version of
@@ -134,6 +133,13 @@ module MergeRequests
       end
     end
 
+    def refresh_pipelines_on_merge_requests
+      merge_requests_for_source_branch.each do |merge_request|
+        create_pipeline_for(merge_request, current_user)
+        UpdateHeadPipelineForMergeRequestWorker.perform_async(merge_request.id)
+      end
+    end
+
     def reset_merge_when_pipeline_succeeds
       merge_requests_for_source_branch.each(&:reset_merge_when_pipeline_succeeds)
     end

changelogs/unreleased/prevent-running-mr-pipelines-when-target-updated.yml

+---
+title: Create pipelines for merge requests only when source branch is updated
+merge_request: 26921
+author:
+type: fixed

spec/services/merge_requests/refresh_service_spec.rb

@@ -146,7 +146,10 @@ describe MergeRequests::RefreshService do
         stub_ci_pipeline_yaml_file(YAML.dump(config))
       end
 
-      subject { service.new(@project, @user).execute(@oldrev, @newrev, 'refs/heads/master') }
+      subject { service.new(project, @user).execute(@oldrev, @newrev, ref) }
+
+      let(:ref) { 'refs/heads/master' }
+      let(:project) { @project }
 
       context "when .gitlab-ci.yml has merge_requests keywords" do
         let(:config) do
@@ -162,14 +165,17 @@ describe MergeRequests::RefreshService do
         it 'create detached merge request pipeline with commits' do
           expect { subject }
             .to change { @merge_request.merge_request_pipelines.count }.by(1)
-            .and change { @fork_merge_request.merge_request_pipelines.count }.by(1)
             .and change { @another_merge_request.merge_request_pipelines.count }.by(0)
 
           expect(@merge_request.has_commits?).to be_truthy
-          expect(@fork_merge_request.has_commits?).to be_truthy
           expect(@another_merge_request.has_commits?).to be_falsy
         end
 
+        it 'does not create detached merge request pipeline for forked project' do
+          expect { subject }
+            .not_to change { @fork_merge_request.merge_request_pipelines.count }
+        end
+
         it 'create detached merge request pipeline for non-fork merge request' do
           subject
 
@@ -177,11 +183,25 @@ describe MergeRequests::RefreshService do
             .to be_detached_merge_request_pipeline
         end
 
-        it 'create legacy detached merge request pipeline for fork merge request' do
-          subject
+        context 'when service is hooked by target branch' do
+          let(:ref) { 'refs/heads/feature' }
 
-          expect(@fork_merge_request.merge_request_pipelines.first)
-            .to be_legacy_detached_merge_request_pipeline
+          it 'does not create detached merge request pipeline' do
+            expect { subject }
+              .not_to change { @merge_request.merge_request_pipelines.count }
+          end
+        end
+
+        context 'when service runs on forked project' do
+          let(:project) { @fork_project }
+
+          it 'creates legacy detached merge request pipeline for fork merge request' do
+            expect { subject }
+              .to change { @fork_merge_request.merge_request_pipelines.count }.by(1)
+
+            expect(@fork_merge_request.merge_request_pipelines.first)
+              .to be_legacy_detached_merge_request_pipeline
+          end
         end
 
         context 'when ci_use_merge_request_ref feature flag is false' do